Coherent light generators – Particular active media – Semiconductor
Reexamination Certificate
2002-07-31
2004-03-16
Ip, Paul (Department: 2828)
Coherent light generators
Particular active media
Semiconductor
C438S039000
Reexamination Certificate
active
06707835
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor laser element including an S-ARROW (Simplified Antiresonant Reflecting Optical Waveguide) structure, in which light in a fundamental transverse mode is confined within a width determined by a pair of elongated high-refractive-index layers separated from each other. The present invention also relates to a process for producing a semiconductor laser element including an S-ARROW structure.
2. Description of the Related Art
Currently, semiconductor laser elements are widely used as light sources for optical communication, optical disk devices, and the like, since laser light can be converged to a diffraction limit. However, only a portion of light emitted from each semiconductor laser element which is in phase at a light-emission end facet can be converged to a diffraction limit. The emission of light in phase at a light-emission end facet of a semiconductor laser element is called emission in a fundamental transverse mode. On the other hand, when light components having various phases coexist (i.e., light components oscillating in higher harmonic transverse modes are mixed) in light emitted from a semiconductor laser element, the light cannot be converged to a diffraction limit.
It is known that the operation in a fundamental transverse mode is more stable when an emission cross section is smaller since the mixture of higher harmonic transverse modes can be more effectively suppressed when the emission cross section is smaller. Therefore, the waveguides in the semiconductor laser elements which are required to emit light in a fundamental transverse mode are arranged to have a thickness of 1 micrometer or smaller and a width of about 2 to 4 micrometers. In particular, it is empirically known that the yield rates of the semiconductor laser elements which stably emit laser light in a fundamental transverse mode increase when the widths of the waveguides are reduced.
Nevertheless, when the emission cross section of a semiconductor laser element is reduced by reducing the width of the waveguide in the semiconductor laser element, the optical density at the light-emission end facet of the semiconductor laser element necessarily increases. The increase in the optical density causes deterioration of the constituent materials of the semiconductor laser element, and becomes a factor which decreases the lifetime of the semiconductor laser element.
In other words, the stabilization of the fundamental transverse mode (by reduction of the cross section of the waveguide) is in a trade-off relationship with the increase in the optical output since it is necessary to increase the cross section of the waveguide for increasing the optical output. Therefore, overcoming this problem is a significant challenge in the current research and development of semiconductor laser.
As an attempt to overcome the above problem, an S-ARROW structure is proposed, as disclosed by H. Yang et al. in “High-power single-mode simplified antiresonant reflecting optical waveguide (S-ARROW),” IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 10, No. 8, August 1998, pp. 1079-1081. Although this structure realizes emission in a fundamental transverse mode, the width of the emission cross section (the lateral width of the waveguide) can be increased to about 6 micrometers, which is 1.5 to 2 times greater than those in the conventional structures. Therefore, when the semiconductor laser element having an S-ARROW structure is used, it is possible to expect increase in the maximum optical output.
The reasons why the semiconductor laser elements having an S-ARROW structure can emit light in a fundamental transverse mode are explained below.
FIG. 10A
shows a cross section (which is perpendicular to the laser propagation direction) of an essential portion of a semiconductor laser element having an S-ARROW structure. In the semiconductor laser element of
FIG. 10A
, a lower cladding layer
38
made of n-type InGaP, an SCH (separate confinement heterostructure) layer
37
being made of InGaAsP and including an InGaAs quantum-well active layer, upper cladding layers
36
and
32
made of p-type InGaP, an etching stop layer
35
made of n-type GaAs, a current stopping layer
34
made of n-type AlInP, guide portions
33
being made of n-type GaAs and having a thickness of, for example, 0.25 micrometers, and a contact layer
31
made of p-type GaAs are formed on an n-type GaAs substrate
39
.
GaAs, which is used for forming the guide portions
33
, has a relatively high refractive index compared with those of the constituent materials around the guide portions
33
. Therefore, the equivalent refractive index in the direction parallel to the SCH layer
37
has a distribution as indicated in FIG.
10
B. That is, the equivalent refractive index in the SCH layer
37
is high under the guide portions
33
, and low in the other regions of the SCH layer
37
.
In the above waveguide structure, the width A of each of the guide portions
33
is determined so that light in the fundamental transverse mode is confined in a region of the SCH layer
37
under the gap between the guide portions
33
, and light in higher harmonic transverse modes is not confined in and leaks out from the region under the gap between the guide portions
33
. In the Yang reference, the width A of each of the guide portions
33
is 0.85 micrometers, and the width B of the gap between the guide portions
33
is 6.5 micrometers.
Due to the provision of the current stopping layer
34
, current which is supplied for generating laser is injected into only the region under the gap between the guide portions
33
, and realizes laser gain in only the region under the gap between the guide portions
33
.
Thus, only the light in the fundamental transverse mode is confined in the region under the gap between the guide portions
33
, and it is possible to obtain a sufficient gain in the fundamental transverse mode. On the other hand, the light in higher harmonic transverse modes is not confined in the region under the gap between the guide portions
33
. Therefore, it is impossible to obtain a substantial gain in higher harmonic transverse modes. As a result, the light in the fundamental transverse mode is dominantly emitted from the semiconductor laser element of
FIG. 10A
, and the semiconductor laser element of
FIG. 11A
can operate in a stable fundamental transverse mode even when the output power is high.
Nevertheless, when the S-ARROW structure is manufactured by the conventional technique, the yield rate is necessarily lowered for the following reasons. In order to explain the reasons, first, the conventional process for producing the semiconductor laser element including the S-ARROW structure is explained with reference to
FIGS. 11
to
14
.
First, as illustrated in
FIG. 11
, the n-type InGaP lower cladding layer
38
, the SCH layer
37
being made of InGaAsP and including the InGaAs quantum-well active layer, the p-type InGaP upper cladding layer
36
, the n-type GaAs etching stop layer
35
, the n-type AlInP current stopping layer
34
, and an n-type GaAs guide layer
33
′ having a thickness of 0.25 micrometers are formed in this order on the n-type GaAs substrate
39
by organometallic vapor phase epitaxy.
Next, in a first photolithography step and an etching step, the outside portions of the n-type GaAs guide layer
33
′ are removed so as to leave a portion of the n-type GaAs guide layer
33
′ including the guide portions
33
in the semiconductor laser element of FIG.
10
A. Thus, a layered structure having a cross section as illustrated in
FIG. 12
is obtained.
Then, in a second photolithography step, as illustrated in
FIG. 13
, a resist pattern
40
is formed over the layered structure of
FIG. 12
except for an area corresponding to the gap (having the width B) between the guide portions
33
in the semiconductor laser element of FIG.
10
A.
Thereafter, the layered structure of
FIG. 13
is etched from the upper side until the p-type InGaP upper cladding
Fuji Photo Film Co. , Ltd.
Ip Paul
Vy Hung T
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